A new smtp protocol with: - mandatory encryption, mandatory validation of certificates - certainty on the sender (effectively a mandatory hardfail spf) - ideally decoupling the email address from the provider running the mail server (users shouldn't be prisonners of a gmail or a yahoo)

[edit] also a system of extended validation for signatures like for ssl certificates would be useful. I would end up expecting a nice green logo in my mail client for emails from citibank.com, something cit1bank.com could not achieve.

[edit2] also a modern design would require emails to be instantaneous. All these messenger apps would become redundant and we would revert to a common standard.